Despite its global impact on ecosystems, the Triassic/Jurassic boundary event had only a modest effect on the carbonate depositional systems of the Southern Alps, whereas a fundamental reorganization of the same palaeogeographic area took place during the Sinemurian Age. This paper investigates whether or not the well-documented demise of Sinemurian carbonate platforms in the Tethyan region was a response to a global event by examination of carbon-isotope anomalies in successions of different facies that record this interval of time. A chemostratigraphic transect from LakeGardauptotheeasternItalianborderisillustrated byfourstratigraphicsections;high-resolution (20 cm over key intervals) chemostratigraphic sampling allowed detection of a major negative δ13C anomaly of 1.5‰, preceded by a positive excursion, both in shallow- and deep-water successions, over the stratigraphical range of the ammonite genus Arnioceras. A comparison with sections from the UK suggests that the positive excursion belongs to the turneri Zone and the succeeding negative excursion falls within the obtusum Zone. In the deep-water Belluno Basin, the negative anomaly occurs in a biogenic chert-rich unit recording the onset of mesotrophic conditions in the basin. In the platform-carbonate successions, this major negative carbon-isotope excursion is developed within a calcarenitic unit corresponding to the lowest occurrence of the foraminifer Paleomayncina termieri. Thisevidencefordeepeningandtransgressionacrossthecarbonateplatformsuggestspre-conditioning for drowning. Hence, rather than tectonic subsidence alone, environmental factors may have aided the demise of Tethyan carbonate platforms during the Early Jurassic Sinemurian Age.
Carbon-isotope anomalies and demise of carbonate platforms in the Sinemurian (Early Jurassic) of the Tethyan region: Evidence from the Southern Alps (Northern Italy) / Masetti, Daniele; Figus, Billy; Jenkyns, Hugh C.; Barattolo, Filippo; Mattioli, Emanuela; Posenato, Renato. - In: GEOLOGICAL MAGAZINE. - ISSN 0016-7568. - 154:3(2017), pp. 625-650. [10.1017/S0016756816000273]
Carbon-isotope anomalies and demise of carbonate platforms in the Sinemurian (Early Jurassic) of the Tethyan region: Evidence from the Southern Alps (Northern Italy)
Barattolo, FilippoMembro del Collaboration Group
;
2017
Abstract
Despite its global impact on ecosystems, the Triassic/Jurassic boundary event had only a modest effect on the carbonate depositional systems of the Southern Alps, whereas a fundamental reorganization of the same palaeogeographic area took place during the Sinemurian Age. This paper investigates whether or not the well-documented demise of Sinemurian carbonate platforms in the Tethyan region was a response to a global event by examination of carbon-isotope anomalies in successions of different facies that record this interval of time. A chemostratigraphic transect from LakeGardauptotheeasternItalianborderisillustrated byfourstratigraphicsections;high-resolution (20 cm over key intervals) chemostratigraphic sampling allowed detection of a major negative δ13C anomaly of 1.5‰, preceded by a positive excursion, both in shallow- and deep-water successions, over the stratigraphical range of the ammonite genus Arnioceras. A comparison with sections from the UK suggests that the positive excursion belongs to the turneri Zone and the succeeding negative excursion falls within the obtusum Zone. In the deep-water Belluno Basin, the negative anomaly occurs in a biogenic chert-rich unit recording the onset of mesotrophic conditions in the basin. In the platform-carbonate successions, this major negative carbon-isotope excursion is developed within a calcarenitic unit corresponding to the lowest occurrence of the foraminifer Paleomayncina termieri. Thisevidencefordeepeningandtransgressionacrossthecarbonateplatformsuggestspre-conditioning for drowning. Hence, rather than tectonic subsidence alone, environmental factors may have aided the demise of Tethyan carbonate platforms during the Early Jurassic Sinemurian Age.File | Dimensione | Formato | |
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